language js

`const $name = new $constructor($callback)` as $stmt where {
  or { $callback <: `() => $body`, $callback <: `function() { $body }` },
  $body <: contains `$name`,
  register_diagnostic(span=$stmt, message="Avoid referencing a variable from a callback inside its own initializer; declare it first with an explicit type.", severity="error")
}
